The current invention is generally related to a method and a system for improving the resolution of color image data in image reproduction machines such as a digital camera and a digital scanner, and more particularly related to the method and the system for independently estimating an intensity value based upon chromaticity values as well as measured sensor values.
In image reproduction machines including a digital camera and a digital scanner, an image photo sensor such as a capacitor coupled device (CCD) includes a group of color-component specific photo elements. These color-component specific photo elements are arranged in a predetermined spatial pattern on a single plane. For example, FIG. 1A illustrates a one-dimensional photo sensor element strip, and a set of red (R), green (G) and blue (B) color-component specific photo sensitive elements is repeated in one dimension. FIG. 1B illustrates a two-dimensional photo sensor, and one exemplary spatial pattern unit of RGB photo elements each consists of five G-sensitive photo elements, two R-sensitive photo elements and two B-sensitive photo elements. These spatially distributed RGB photo sensors generate an image for respective pixels.
Now referring to FIG. 2, the above described one-dimensional single-layer RGB photo sensor array unit generates only one color value for the corresponding CCD or CCD data. The CCD data is processed to ultimately generate three sets of color values for each pixel or CCD in a color image. In particular, the one-dimensional single-layer RGB photo sensor array unit initially generates spatially distributed RGB values or CCD data. For example, since R-sensitive photo sensor elements are located at the first and fourth positions in the array unit, the R CCD data is available only at these two positions. Similarly, G CCD data is available only at the second and fifth positions while B CCD data is available at the third and sixth positions. Based upon the above described fragmented CCD data, the three sets of contiguous color values are generated according to a predetermined process such as interpolation for an improved color image.
The above described image photo sensor units generally have two undesirable features. One of the undesirable features is caused by the spatial location of the RGB photo elements on a single plane. Since the RGB photo sensors are not stacked on top of each other at an identical location with respect to an object image to be reproduced, an exact location of the object image that each of these RGB photo elements reproduces is not identical. In other words, each of the RGB elements generates a slightly different portion of the object image. Because of this spatial distribution of the photo sensors, the reproduced object image is somewhat distorted in its colors.
The other undesirable feature is related to resolution. Since color at one pixel is determined based upon a set of color-component specific photo elements such as a set of RGB elements, the resolution of the reproduced image is reduced by a number of photo elements required for one pixel in determining color. In contrast, if an achromatic image is reproduced, since each pixel directly corresponds to a portion of an output image, the resolution is directly related to the number of photo elements.
To solve the above described problems, prior attempts include Japanese Laid Open Publication Hei 2-153679 which discloses a method of interpolating pixel color output values. Referring to FIG. 3, the R color-component values are interpolated in a one-dimensional single plane photo sensor strip as shown in FIG. 1A. Since the R photo sensors exist only at positions 1 and 4, the R values are interpolated at positions 2 and 3 based upon the R output values at the positions 1 and 4. The following equations (1):                                           R            ⁡                          (              2              )                                =                                                    2                ⁢                                  R                  ⁡                                      (                    1                    )                                                              +                              R                ⁡                                  (                  4                  )                                                      3                          ⁢                  
                ⁢                              R            ⁡                          (              3              )                                =                                                    R                ⁡                                  (                  1                  )                                            +                              2                ⁢                                  R                  ⁡                                      (                    4                    )                                                                        3                                              (        1        )            
Similar interpolation is performed for other color components. Although the above interpolation method somewhat improves color distortion, because the interpolated values are always within a range of actually measured values, the improved output range is still limited to the measured range.
Another prior art attempt, Japanese Laid Open Publications Hei 2-239791 and Hei 7-123421 disclose methods for increasing resolution in image reproduction. The methods assume that a set of color-component specific photo sensors each has identical response sensitivity to achromatic light and that an image is achromatic. The intensity output from each photo sensor is now used for generating an image thereby increasing resolution. For example, referring to FIG. 4 even though individual photo elements are color-component specific such as R, G and B, only monochromatic intensity is considered. However, when the sensitivities are not identical and or the image is not achromatic, the sensitivities may be adjusted for a particular color component if relative color-component sensitivity response curves of the photo elements are known. The above described approach is useful only for a single color-component output and is not practical for chromatic images in general.
In order to avoid the above described unresolved problems in improving the resolution of an image generated by the single-plane color photo sensor, it is desired to process photo sensor signals according to a predetermined spatial distribution pattern of the color-component. Furthermore, it remains desirable to process the photo sensor signals in such a way to generate intensity signals which substantially improve the above described color distortion problem.
In order to solve the above and other problems, according to a first aspect of the current invention, a method of improving resolution in color image data, includes the steps of: a) inputting color image data from a sensor having a specific unit pattern of color-component specific photo elements; b) generating chroma values for each of the color-component specific elements from the color image data; c) adjusting the chroma values according to the specific pattern of the color-component specific elements; and d) estimating an intensity value based upon the chroma values adjusted in the step c) and the color image data from the step a).
According to a second aspect of the current invention, a system for improving resolution in color image data, includes: a single-plane color image sensor having a predetermined spatial pattern of color-component specific photo elements for generating color image data; an interpolated chroma value generator connected to said single-plane color image sensor for generating interpolated chroma values according to the spatial pattern; and an intensity estimator connected to the interpolated chroma value generator and the single-plane color image sensor for estimating an intensity value based upon the interpolated chroma values and the color image data.
According to a third aspect of the current invention, a program storage device readable by a machine, tangibly embodying a program of instructions executable by the machine to perform method steps of reproducing a high-resolution image, the method steps including: j) inputting color image data from a sensor having a specific unit pattern of color-component specific photo elements; k) generating chroma values for each of the color-component specific elements from the color image data; l) adjusting the chroma values according to the specific pattern of the color-component specific elements; m) further adjusting the chroma values for smoothing the chroma values adjusted in the step l); n) estimating an intensity value based upon the chroma values twice adjusted in the steps l) and m) and the color image data from the step j); o) adjusting the intensity value for an improved edge characteristic after the step n); and p) generating RGB data based upon the chroma values adjusted in the step m) and the intensity value adjusted in the step o).
These and various other advantages and features of novelty which characterize the invention are pointed out with particularity in the claims annexed hereto and forming a part hereof. However, for a better understanding of the invention, its advantages, and the objects obtained by its use, reference should be made to the drawings which form a further part hereof, and to the accompanying descriptive matter, in which there is illustrated and described a preferred embodiment of the invention.